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Tube Regulator

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I’m building a pre-amp using 12AT7. In the regulator section (after rectifier) there will be 2 tubes, one of them is a 12AT7. Can I replace the 12AT7 with 12AX7?

12AT7 max. plate current = 15mA
12AT7 max. plate dissipation = 2.5W

12AX7 max. plate current = 6mA
12AX7 max. plate dissipation = 1.1W
 
Hmmmm….

<B>Fdegrove</B>, I think I will need to think hard to calculate the required current for the pre-amp :bawling: The transformer is 300V-CT-300V. I think it works similarly with <B>siu sin man tho</B> pre-amp.

I just thought that 6mA maximum plate current won’t be a problem for a common pre-amp circuit. What if I just try the 12AX7 and see if no red light emitted from the 12AX7 and no degradation in the sound quality? I have plenty of 12AX7.

I plan to buy (for 1 channel) three Mullard CV4024 and 1 Chinese (or RCA) 12AT7 to experiment with types (before buying for 2 channels)

Cheers,:)
 
diyAudio Senior Member
Joined 2002
IF ONLY...

Hi,

Fdegrove, I think I will need to think hard to calculate the required current for the pre-amp The transformer is 300V-CT-300V. I think it works similarly with siu sin man tho pre-amp.

Well, if we had an idea about this regulators' composition it would be great help to you.
Short of that it's rather pointless.

Cheers,;)
 
Vacuum regulators

I have seen gaseous regulators such as 0C3

I have seen triodes used as regulators such as the 6080.

I think it is posible to design a good stable power supply for vacuum tube amplifiers. The use of choke input filters is a must.

Some folks are hung up on vacuum rectifiers. Vacuum rectifiers present poor regulation. My advice is to say goodbye to 5U4s etc. I have heard all sorts of arguments for vacuum regulation. With a properly designed filter, switching noise is not an issue.
 
Re: Vacuum regulators

valveluver said:
Some folks are hung up on vacuum rectifiers. Vacuum rectifiers present poor regulation.

Not in class A PP or differential circuits.

My advice is to say goodbye to 5U4s etc. I have heard all sorts of arguments for vacuum regulation. With a properly designed filter, switching noise is not an issue.

I don't agree. It's not hard to see it on a spec-an, and snubbers are only effective for a given load. As the duty cylcle changes with load (say class AB), so does the hash spectrum. PSU chokes don't do much at all to filter switching noise, not do most PSU caps.

There are better diodes than the 5U4 etc.
 
<i>"Well, if we had an idea about this regulators' composition it would be great help to you. Short of that it's rather pointless."</i>

Well, I think you are right. But not 100% ;)

The pre-amp is SE (I haven’t had a chance to have a second look on the circuit). Hence there will be an important voltage ripple due to changing current in the load. How much current or voltage maximum required by 12AT7 pre-amps? I don’t know. With 300V transformer, I think the output will be around 200V? I noticed a potentiometer in the regulator, so this may be to set the output voltage (so it’s quite variable).

If the 12AT7 acted as regulator, the maximum plate current and power dissipation is important thing to notice. I noticed a 300V transformer and 400V capacitor, so the output after the bridge may be 350V. If the potentiometer can adjust the voltage to say 200V as the minimum pre-amp requirement (regardless the current requirement), the voltage drop will be 150V (maximum, because other components are likely to drop the voltage instead of increase the voltage; I didn’t see a big cap in the output). With 150V voltage drop, and the ability to dissipate about 2W, the current will be around 2/150, less than 20mA. From the maximum value of 12AT7 plate current itself (15mA), I can’t imagine a pre-amp requires such a low current (I don’t know about tubes).

Thus, the 12AT7 must not act as a regulator? So what? Error amplifier? For error amplifiers, current and power capability is not important. The important thing is the gain and the ability to handle an expectedly wide range of voltage, right? The 12AT7 and 12AX7 shows equally the same maximum voltage (so I didn’t write it on my first post), but different gain. The 12AT7 is 70% (or 60%?) of 12AX7 gain, so why is 12AT7 chosen for this circuit? Is 12AT7 cheaper than 12AX7?

<i>"I think it is posible to design a good stable power supply for vacuum tube amplifiers. The use of choke input filters is a must."</i>

Of course it is possible ;) But choke is a must??
 
diyAudio Senior Member
Joined 2002
RE:REGULATOR.

Hi,

In your first post you state you are building a preamp using the 12AT7 in the amplifying stages plus an two tubed regulator.
One 12AT7 plus another tube unknown to us.

I assume the 12AT7 is used as an error amplifier, you than ask if you could use a 12AX7 instead of this.

Again assuming the above is correct, my answer would be : yes, you can provided you adjust some values so you can take advantage of the higher mu of the 12AX7 otherwise it is rather pointless.

That is what I meant in post # 6.

Also, you say you have a 300V CT xformer, depending on how you intend to rectify that voltage it could give you a B+ of as much as 800V which is of course way too much for your preamp.

So, for a series regulator using all tubes you would need a series pass device (a trioded EL84/6BQ4 e.g.), a 12AX7 as an error amp and a voltage reference such as an 85A2/5651.

Hope it helps,;)
 
<I>”Also, you say you have a 300V CT xformer, depending on how you intend to rectify that voltage it could give you a B+ of as much as 800V which is of course way too much for your preamp”</I>

I don’t know how tube rectifier works, but it can’t be 800V as the rating for the supply caps is 400V ;)

The other tube is something like 12B4A, I don’t remember. With solid-state regulator, I know that brutal change of component values may lead to loosing regulation function. Here I don’t have oscilloscope to tweak the regulator, so I will do it by ear (quite hard, huh?). But the most important thing I want to know is the risk… (like this one below)

I have old caps that I’m using for 70V Bride Of Zen pre-amp. I want to use them for my tube pre-amp (CRC after the rectifier). The voltage rating is OK. If these caps are blown, will the tubes in the pre-amp be damaged? I think it is very unlikely, but I’m not sure (I may use expensive tubes in the pre-amp)

Thaks, Frank!
 
Solid state rectification

If there is a problem with solid state rectifiers, can anyone explain the noise that I am supposed to see or hear?

A 10 henry choke has a reactance of +j. 7500 at 120 hertz. If the noise occurs above 120 hz the reactance is even higher.

A 100 mirofarad capacitor has a reactance of -j. 13.2 at 120 hz. and is even lower above 120 hz.

How could such a noise make it through the filter?
 
...Bedtime reading...I need to get a life...

Hi Frank,

That's a great thread, I've enjoyed reading and learned a bit.

As someone who has used 1n4007 for a while and had good success applying snubbers to them to quiet them down - I was surprised at the difference BYV26E ultrafast soft recovery diodes made ( Benny Glass recommended and suplied). I actually rate them better than using 5U4 or GZ34... That may just be lucky in that the BYV didn't excite the parasitics but the valve diodes did...

Joel,

It not just Frank there are lots of us who can hear these differences - It's just Frank who has his head above the parapet:clown:

ciao

James
 
<i>"If there is a problem with solid state rectifiers, can anyone explain the noise that I am supposed to see or hear?
A 10 henry choke has a reactance of +j. 7500 at 120 hertz. If the noise occurs above 120 hz the reactance is even higher.
A 100 mirofarad capacitor has a reactance of -j. 13.2 at 120 hz. and is even lower above 120 hz.
How could such a noise make it through the filter?"</i>

I don’t know about this, but I think you should be able to see it in an oscilloscope if the noise does occur. (I have never used oscilloscope in audio, just for other purposes in university). Can we hear it by ear? If the noise does occur, why not?

From my experience with power supply regulator, I prefer to call it “hum”, not “noise”. The effect is in the bass performance. It is about 100Hz right? (some countries have 50Hz instead of 60Hz) When the bass had become tighter and clear, I assumed the “hum” had gone.

As for LRC filtering (never used an inductor), calculation on paper had never been accurate. I used that value only as a base value and then tweak it by ear. I couldn’t believe that I could hear such thing :confused: :confused: :confused:

I don’t really care if SS rectifiers are more noisy than tubes. If I had a chance to have both types of regulators, I will try to hear it and choose the one I prefer. It could be the rectifiers, could be just the “wrong” brand, could be something else, but you cannot blame my ears. They are the only measurement tools I have :(
 
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